The present invention relates to an electrical method of imaging sub-surface structures within a body having differing electrical impedances, by measurement of surface or internal potentials caused by impressed currents applied externally or internally to the body. The term "impedance" is to be understood in the generic sense, relating to the ratio from time to time of voltage to current during transient signal responses (e.g. pulses) as well as during steady-state signal responses.
Electrical impedance methods have been used in geological and mineral prospecting applications. For a summary of typical methods used in these applications, reference may be made to the textbook Applied Geophysics by Telford, Geldart, Sheriff and Keys, published by Cambridge University Press (1976). At page 632 of this text, it is pointed out that all resistivity (or, more generally, "impedance") methods employ an artificial source of current which is introduced into the ground through point electrodes or long line contacts. The procedure then is to measure potentials at other electrodes in the vicinity of the current flow. In most cases, the current is also noted; it is then possible to determine an effective or apparent resistivity of the subsurface. The authors further stated:
"In this regard the resistivity technique is superior, theoretically at least, to all the other electrical methods, since quantitative results are obtained by using a controlled source of specific dimensions. Practically--as in the other geophysical methods--the maximum potentialities of resistivity are not usually realized. The chief drawback is its large sensitivity to minor variations in conductivity near the surface; in electronic parlance the noise level is high. An analogous situation would exist in magnetics if one were to employ a magnetometer with sensitivity in the milligamma range. This limitation, added to the practical difficulty involved in dragging several electrodes and long wires over rough wooded terrain, has made the electromagnetic method more popular than resistivity in mineral exploration. Nor is resistivity particularly suitable for oil prospecting."
Thus, while the theoretical possibilities and advantages of resistivity techniques are recognized by Telford et al, the practical shortcomings of typical embodiments of such techniques are stated to be sufficiently serious that electromagnetic methods are more popular--at least in mineral exploration--and of course, they also state that "resistivity" is not particularly suitable for oil prospecting.
Electrical impedance methods have also been used in the medical field to measure certain overall cardiac parameters, intrathoracic fluid volumes, etc. Examples of such methods are disclosed in U.S. Pat. Nos. 3,750,649 (Severinghaus) issued Aug. 7, 1973; 3,294,084 (Schuler et al) issued Dec. 27, 1966; 3,452,743 (Rieke) issued July 1, 1969; 3,608,543 (Longini et al) issued Sept. 28, 1971; 3,835,840 (Mount) issued Sept. 17, 1974; 3,874,368 (Asrican) issued Apr. 1, 1975; 3,996,924 (Wheeler) issued Dec. 14, 1976; and 3,980,073 (Shaw) issued Sept. 14, 1976. However, none of the foregoing references deal with imaging of internal organs but merely with measurement of overall cardiac parameters.
Recently, computer processing of X-ray absorption data has been used to produce images of internal organs. One of the most active companies in the use of computerized axial tomography has been E.M.I. Ltd., which is the assignee of numerous patents in this field, including U.S. Pat. Nos. 3,847,466 issued Dec. 12, 1974; 4,066,900, 4,066,902, 4,066,903 and 4,066,906 issued Jan. 3, 1978; 4,070,581 issued Jan. 24, 1978; 4,071,760 issued Jan. 31, 1978; 4,072,875 issued Feb. 7, 1978; 4,075,700 issued Feb. 21, 1978; 4,076,985 issued Feb. 28, 1978; 4,081,681 issued Mar. 28, 1978; 4,084,093 and 4,084,094 issued Apr. 11, 1978; 4,088,887 issued May 9, 1978; 4,091,285; 4,091,286; 4,091,287 and 4,091,289 issued May 23, 1978; 4,096,390 issued June 20, 1978; 4,097,744 and 4,097,746 issued June 27, 1978; 4,101,768 and 4,101,773 issued July 18, 1978; 4,103,169 issued July 25, 1978; 4,115,691; 4,115,697 and 4,115,698 issued Sept. 19, 1978; and 4,117,366 issued Sept. 26, 1978. Another company active in radiation scanning techniques is Ohio Nuclear, Inc., the assignee of various patents in the field, including U.S. Pat. Nos. 3,320,418 isssued May 16, 1967; 3,695,252 issued Oct. 3, 1972; 3,787,827 issued Jan. 22, 1974; 3,908,128 issued Sept. 23, 1975; 3,970,852 issued July 20, 1976; and 4,071,771 issued Jan. 31, 1978. General Electric Company has patents in the field of computerized tomography scanning including U.S. Pat. Nos. 4,115,695 and 4,115,696 issued Sept. 19, 1978. As may be observed from consideration of the foregoing patents, computerized radiation tomography is complex and expensive. The mechanics of the various techniques and equipment employed are large, costly and comparatively slow and they cannot be used to follow dynamic activity of organs. Perhaps most importantly, radiation techniques are hazardous--especially for their long-term effects.
An imaging and reconstruction technique which has been of some interest recently involves the use of ultrasonics. U.S. Pat. No. 4,074,564 of Varian Associates, Inc., issued Feb. 21, 1978, teaches that short bursts of ultrasonic energy may be directed through a three-dimensional specimen to determine the spatial distribution of those structures within the specimen capable of affecting the waveform of the energy. Transducers are placed in spaced positions about the periphery of the specimen to measure the affected parameters (such as attenuation and delay time) of the energy as a result of passing through the specimen along paths between the spaced transducers. The output signals containing this transit time and energy absorption information may be retained in a data storage device. Through conventional programming techniques, a computer processes the data and calculates a velocity or absorption profile for each path. The profiles are collectively used to reconstruct two-dimensional or three-dimensional images of the specimen. Analog reconstruction methods are also used. However effective ultrasonic imaging and reconstruction techniques may be, it is suspected that they suffer from the same general drawback as X-ray or radiation scanning techniques,--i.e. they are hazardous to health. Indeed, it is widely thought that ultrasonic energy absorbed by a living body can cause genetic damage. Also, ultrasonic imaging presents only a restricted field of view.
Unlike X-rays or ultrasonic energy, low-magnitude electric currents are not known to have adverse effects upon animals or humans. For this reason, electrical impedance measurement techniques have been of interest in the medical field, but have been insufficiently researched to be useful in the reconstruction and imaging of internal organs. Henderson, Webster and Swanson in "A Thoracic Electrical Impedance Camera" (29th ACEMB Digest, P. 332) showed that the lung fields can be mapped in spite of their simplified assumption that currents flow in beam-like fashion through the thorax. However, their approach can only produce a surface map influenced by the lung and lung-water distribution. Such results may permit interpretation of physiological phenomena but cannot directly produce tomographic images.
Using the electrical approach it has been proposed by workers in the field to obtain tomographic images, but such workers have invariably been deterred by the fact that the electrical currents injected into the body being imaged do not travel in straight lines but rather spread out and take many different paths. Since this has been perceived as a problem, it has been attempted to utilize "guard electrodes" which surround the primary electrodes and which prevent or at least restrict the spreading out of the current flowing between the primary electrodes--see, for example, the aforesaid U.S. Pat. Nos. 3,750,649; 3,452,743 and 3,608,543. However, the guard ring approach cannot be totally effective to constrain the currents to straight-line flow.
In electrical imaging methods that assume beam-like or straight-line current flow, voltages must be measured at or very near to active electrodes through which current is impressed upon the body. In doing this, there is an implicit and incorrect parallel drawn with X-ray tomography. The assumption that the currents travel as beams--whether straight or curved--does not correspond with actuality and results in consequent errors due to improper modelling of flow path, shape, width and length, that would obscure detail in fine structures--see R. H. T. Bates et al, "A Limitation on Systems for Imaging Electrical Conductivity Distributions"--I.E.E.E. Transactions on Biomedical Engineering, Vol. BME-27, No. 7, July, 1980--pp. 418-420. Moreover, the necessity for measurement of a voltage at an active current electrode causes the inclusion of the effect of contact resistance to degrade measurements that must be accurate in order to produce fine detail. The measurement of voltages very near to an active electrode, because of the steep voltage gradient in the vicinity of the electrode, would also result in significant inaccuracy.